A compact constitutive model to describe the viscoelastic-plastic behaviour of glassy polymers: Comparison with monotonic and cyclic experiments and state-of-the-art models

A number of constitutive models have been developed for solid polymers to describe the large deformation behavior. However, most of the existing models rely on a purely elastic or hyperelastic initial response when they are incapable of accurately predicting the cyclic stress-strain hysteresis loops. In this work, therefore, a compact cyclic viscoelastic-viscoplastic constitutive model is proposed to improve the prediction of the loops below the peak yield stress. The proposed approach is based on the distinguished model by Haward and Thackray (1968) for glassy polymers, which is augmented by a few thermodynamically motivated internal state variables able to predict the missing viscous deformation behavior, including the nonlinear cyclic hardening in three dimensions. Based on the comprehensive uniaxial tension experiments, it is demonstrated that this compact formulation, along with a calibration scheme, enables accurate prediction of the shape of the hysteresis loops, as well as the representation of ratcheting behavior. A comparison is also made with state-of-the-art models that are capable of predicting the cyclic stress-strain hysteresis loops.